This invention related to an improved form of heat exchanger for cryogenic fluids. The use of a heat exchanger results in the relatively rapid warming and vaporization of cryogenic fluids, e.g. liquid nitrogen, liquid oxygen, etc. In practice, the cryogenic fluid is removed from a storage tank and passed via a conduit to the user. While in transit, heat from the ambient air warms the conduit and vaporizes the cryogenic fluid. The resulting vapors are used in a wide variety of industrial applications, including welding, missle fuel tank filling, and the like.
Generally, the heat exchanger takes the form of heat transfer elements or sleeves which surround and closely contact the conduit through which the cryogenic fluid is passing. These sleeves are made from a material having a relatively high thermal conductivity and typically are provided with fins or other extended surfaces in order to increase their surface area, thereby resulting in more rapid vaporization. Most heat exchanger units consist of approximately a dozen or more separate sleeve sections which are arranged in a vertical parallel fashion and which are interconnected by a manifold system so that the cryogenic fluid passes through them in serpentine fashion.
Early heat transfer sections consisted of long, multifined extruded aluminum sections with a hole centrally located and running the length of the section. As the cryogenic fluid passed through the axially extending hole, heat was transferred from the air to the fluid, warming and vaporizing the fluid. A vaporizer unit consisted of a plurality of such heat transfer sections disposed vertically and arranged in a bank. The sections were connected in series, with the conduit welded to the output opening of one section being welded to the input opening of the next section. However, the large number of welds required proved to be a substantial defect of this method since the welds often failed due to the extreme pressure from the cryogenic fluid, the thermal cycling of the vaporizer, and the differential rate of contraction of dissimilar metals. The failure of these welds lead to leakage, loss of pressure and other related problems.
Later vaporizing systems contained the cryogenic fluid in a continuous conduit and surrounded the conduit with heat transfer sections. These methods eliminated the welding proglems of earlier vaporizers but created the new problem of locking the heat transfer sections to the conduit so as to provide effective thermal contact between them. One method used a conduit extending through an axially extending hole in the heat transfer section. The conduit was slightly undersized for the hole, effective thermal contact being effected by expanding the conduit under the large pressure exerted by the cryogenic fluid. This method suffered from leakage problems, lack of good thermal contact and economic unfeasibility.
An alternative system for providing effective thermal contact between the conduit and the heat transfer section uses two identical heat transfer sections which are joined longitudinally around the conduit. Each section has a central cylindrical hub portion for engaging the conduit, a plurality of fins extending radially from the central hub for collecting the heat, and two short locking members disposed on the hub for locking the two heat transfer sections together around the conduit. When the two sections are placed around the conduit, the pair of hub portions join circumferentially around the conduit and the two locking members are in close physical proximity, one pair on each side of the conduit. With the aid of a specially designed machine the longer, deformable locking member is folded over the shorter, adjacent fixed locking member. The longer member extends around the shorter member, thereby holding the two heat transfer sections together around the conduit.
This method suffers from some substantial defects. First, this method does not always provide good thermal contact between the heat transfer sections and the conduit, since the very low temperatures of cryogenic liquids cause the conduit to contract away from the hub portions. As a result, an air or ice gap forms between the conduit and the heat transfer sections, thereby substantially reducing the efficiency of the heat exchanger. Secondly, the heat transfer sections must be assembled by means of a highly particularized, bulky, and expensive machine which sometimes even deforms the heat transfer sections or the conduit. Thus, the conduit must be fitted with the heat transfer sections prior to assembly of the conduit structure. Also, assembly is impossible in the field, and disassembly and reuse of the heat transfer sections is virtually impossible.
In addition, a substantial amount of ice builds up on the heat transfer sections immediately surrounding the central hub portions, thereby preventing the efficient flow of heat from the air to the conduit. This icy build-up is substantially accelerated by the location of the locking members on the hub portions, since these locking members increase the heat transfer surface area in the vicinity of the hubs. As a result, the heat exchanger must be shut down and the fins and hub portions de-iced, resulting in a severe loss of time and increased expense. Another disadvantage resulting from the location of the locking members on the central hubs is that all of the fins cannot extend directly from the center section, thereby increasing the path length which the heat must be conducted in order to reach the conduit.